System for laser photocoagulation of the retina

ABSTRACT

The invention relates to a system for laser photocoagulation of the retina, comprising: a photocoagulation laser (1); an optical path (5) connecting the upstream photocoagulation laser (1) to a downstream laser outlet opening (6) intended to be positioned in front of the retina; an adaptive optical element (9) positioned in the optical path and configured to modify the wavefront of the laser beam being propagated in the optical path, in order to compensate for aberrations of the eye that occur as far as the retina; a position control loop (10) controlling a first actuator (14) positioned in the optical path downstream of the adaptive optical element in order to control the position of the laser outlet opening relative to the retina to be treated; and at least one imaging device (8) configured to obtain an image of the retina diverted from the optical path.

TECHNICAL FIELD OF THE INVENTION

The present invention belongs to the field of ophthalmology. Morespecifically, the invention relates to a system for laserphotocoagulation of a retina.

Macular edema is a quite common retinal disorder characterized by one ofan edema of the central region of the retina (macula) related toabnormalities of small blood vessels, resulting in reduced visualacuity. Macular edema is the main manifestation of diabetic retinopathy,which is the leading cause of blindness before age 55. The standardtreatment for these macular edemas consists in performing aphotocoagulation of these vessels in the central area of the retina witha laser. Other diseases may also benefit from the photocoagulation ofthe macula, such as retinal vein occlusions.

During a photocoagulation, the therapeutic target of the laser may belocated at two layers:

-   -   the layer of the retinal blood vessels, in the case of a        localized edema. In this case, the edema is caused by a vascular        dilation called a macroaneurysm on an identified retinal vessel,        and a direct photocoagulation of the involved vessel is carried        out;    -   the retinal pigment epithelium, in the case of diffuse edema.        One solution then consists in stimulating the absorbing function        of the pigment epithelium, by photocoagulating it with repeated        impacts.

These two layers of the retina are separated by a hundred microns, andare surrounded by other layers corresponding to functional tissues thatmust be preserved, at the risk of causing permanent loss of vision.

Until now, the surgeon operating the laser only had a two-dimensionalvisualization of the retina with a slit lamp, seen from the front, onwhich the surgeon relies to localize the point of laser impact on thesurface of the retina, and dose the power of the laser: it isrecommended to apply the laser until observing a discreet whitening withthe slit lamp. It therefore appears that in the current surgicalprocedures, the focusing and the dosage of the laser are very empirical.Indeed, standard treatment procedures are not very reproducible from onepractitioner to another.

In addition, during the operation, the eye is more or less stabilizedwith a contact lens held on the eye, which filters only partially thecontinual involuntary fixing movements of the eye. As a result, thelaser impact may not correspond to the aimed position on the ocularfundus image obtained by the slit lamp.

To overcome this lack of accuracy in the localization of the laserimpact, the current systems are configured to generate a large-sizedlaser impact, in order to ensure that the photocoagulation completelycovers the area to be treated. The large size of the laser impact isobtained by delivering a broad laser beam, with a lateral diameter ofthe focal spot on the retina ranging from 100 μm to 500 μm, and having asmall optical aperture, with longitudinal extension (in depth) of thefocal spot on the retina of about 300 μm.

Furthermore, a large-sized laser impact also allows overcoming variousocular aberrations reducing the accuracy of the localization of thelaser impact. Indeed, a real human eye is not strictly stigmatic, thatis to say the image of a point is not a strictly clear point. Theseocular aberrations can be static, the most common examples being visiondefects corrected by glasses (myopia, hypermetropia, astigmatism, etc.).Aberrations can also be dynamic, for example caused bymicro-accommodations of the crystalline lens, the tear film flow and theeye movements. These aberrations result in a Point Spread Function (PSF)which deviates from that of a theoretical perfect eye, and which variesrapidly over time.

The large size of the current laser impacts is not adapted to the sizeof the areas to be treated. By way of example, the macroaneurysms havesizes varying typically between 100 μm and 300 μm, even though the laserimpact has a diameter of about 300 μm, not to mention the thermalscattering occurring around the laser impact area. As a result, lesionsappear in the healthy tissues surrounding the area to be treated.However, the layer of the retinal blood vessels and the retinal pigmentepithelium are surrounded by functional tissues whose deterioration canlead to permanent loss of vision.

PRESENTATION OF THE INVENTION

The invention aims at proposing a system for laser photocoagulation of aretina allowing accurate localization of the laser impact, both on thesurface of the retina and in depth, and containing this laser impact ina restricted area, typically smaller than 100 μm, in order to limit thedamage to healthy tissues in the vicinity of the area to be treated.

To this end, a system for laser photocoagulation of a retina isproposed, comprising:

-   -   a photocoagulation laser comprising a laser source and an        optical transport fiber,    -   an optical path connecting the upstream photocoagulation laser        to a downstream laser output intended to be placed in front of        the retina, a laser beam emitted by the photocoagulation laser        being intended to propagate along the optical path,        wherein the laser source is a laser source configured to emit in        a wavelength comprised between 520 nm and 690 nm in the optical        fiber, and in that the optical transport fiber has a core with a        diameter less than 50 μm, the system further comprising:    -   an adaptive optics positioned in the optical path and configured        to modify the wavefront of the laser beam propagating in the        optical path in order to compensate ocular aberrations occurring        up to the retina,    -   a position control loop controlling a first actuator positioned        in the optical path downstream of the adaptive optics to        servo-control the position of the laser output with respect to        the retina to be treated,    -   at least one imager configured to acquire an image of the retina        derived from the optical path.

The system is advantageously completed by the following characteristics,taken atone or in any one of their technically possible combination:

-   -   a diameter of the laser beam at the pupil plane of the laser        output 6 is comprised between 4 mm and 8 mm;    -   a beam angle at the laser output is less than 5°;    -   the control loop comprises two control sub-loops:        -   a first control sub-loop comprising a first imager            configured to receive an image of the retina derived from            the optical path downstream of the first actuator according            to a first acquisition field,        -   a second control sub-loop comprising a second imager            configured to receive an image of the retina derived from            the optical path upstream of the adaptive optics according            to a second acquisition field.    -   the first field has a field angle at least twice as great as a        field angle of the second field, and/or the second control        sub-loop operates at a frequency greater than a frequency of the        first sub-loop;    -   the system further comprises a visualization subsystem        configured to receive an image of the retina derived from the        optical path downstream of the first actuator and acquire images        according to the first field;    -   the system further comprises a second actuator placed upstream        of the adaptive optics, configured to receive a laser        displacement command, and to act on the path of the laser beam        in order to move the laser impact in three distinct directions        in space;    -   the system is configured so that the second actuator moves the        path of the laser beam while the photocoagulation laser emits        the laser beam, such that the laser impact on the retina moves        between several locations in a continuous manner;    -   the second actuator comprises:        -   a scanner positioned in the optical path and adapted to            perform a displacement of the laser beam in a focal plane            perpendicular to the propagation of the laser beam, and        -   a connector positioned in the optical path and movable in            controlled translation in the direction of the laser beam;    -   the imager comprises a camera configured to acquire an image of        the retina derived from the optical path upstream of the        adaptive optics, the camera being movable in controlled        translation in the direction of propagation of the light        received by said camera;    -   the adaptive optics comprises:        -   a correcting element, disposed on the optical path,        -   a processing module configured to control the correcting            element based on an analysis of a wave surface of the light            traveling over the optical path;    -   the analysis of the wave surface is provided from a wave front        sensor configured to receive light from the optical path        upstream of the correcting element and to determine a        measurement representative of a shape of a wave surface of the        light traveling along the optical path, or the imager comprises        a camera configured to acquire an image of the retina derived        from the optical path upstream of the adaptive optics, or the        analysis of the wave surface is provided from an image acquired        by the imager comprising a camera configured to acquire an image        of the retina derived from the optical path upstream of the        adaptive optics;    -   the system comprises an optical coherence tomography device        connected to the optical path downstream of the adaptive optics;    -   the laser source is a single-mode fiber laser source.

PRESENTATION OF THE FIGURES

The invention will be better understood, thanks to the descriptionbelow, which relates to embodiments and variants according to thepresent invention, given by way of non-limiting, examples and explainedwith reference to the appended schematic drawings, in which:

FIG. 1 schematically illustrates the main components of a system forlaser photocoagulation of a retina according to one embodiment;

FIG. 2 illustrates a configuration in accordance with the oneillustrated in FIG. 1 .

DETAILED DESCRIPTION

With reference to FIG. 1 which schematically represents the mainfunctional elements of the system, the system for laser photocoagulationof a retina comprises a photocoagulation laser 1 comprising a lasersource 2 and an optical transport fiber 3. The laser source 2 ispreferably a fiber laser source, that is to say whose amplifying mediumis created in a rare-earth doped optical fiber. This optical fiber has acore diameter of preferably less than 12 μm, and more preferably lessthan 8 μm, such as for example 6 μm. The laser source 2 is said to besingle-mode source that is to say with a parameter M² less than 1.5, andpreferably less than 1.2. The single-mode aspect and the small diameterof the optical fiber of the laser source 2 allows obtaining a lowcoupling loss (less than 30%) during the transmission of the laser beamin the optical transport fiber 3 of the photocoagulation laser 1.

The laser source 2 is configured to emit in a wavelength comprisedbetween 520 nm and 690 nm in the optical transport fiber 3, andpreferably in a wavelength comprised between 540 nm and 630 nm, and morepreferably between 550 nm and 600 nm. The laser source 2 is configuredto emit a laser beam having sufficient power to cause photocoagulationof the targeted area of the retina 4, and in particular to cause therequired thermal effect. For example, the laser beam can have a powercomprised between 50 mW and 3 000 mW.

The optical transport fiber 3 receives the laser beam emitted by thelaser source 2 and allows this laser beam to propagate in a non-rectilinear fashion over a long distance without risk. The optical transportfiber 3 thus allows spatial decoupling between the photocoagulationlaser 1 and the rest of the system, and therefore allows moving thelaser source several meters from the retina 4. The optical transportfiber 3 has a core diameter of less than 50 μm, and preferably less than25 μm, more preferably less than 15 μm, such as for example 12.5 μm. Asa result, this transport fiber 3 can be spatially single-mode orslightly multimode. Preferably, the transport fiber 3 has a parameter M²less than 1.5, and preferably less than 1.2.

Furthermore, it is the output of this optical transport fiber 3 of thephotocoagulation laser 1 that is imaged through the rest of the systemon the patient's retina. The small diameter of the optical transportfiber 3 results in a small-dimensioned focal point, improving thecontainment of the laser impact. Thus, the system is characterized by adiameter of the laser beam at the pupil plane (that is to say at theposition where the patient's pupil is positioned) of the laser output 6comprised between 4 mm and 8 mm. Preferably, the system is alsocharacterized by a low numerical aperture at the laser output 8. Thenumerical aperture in the air corresponds to the sine of the halfaperture angle, which is the angle between the optical axis and the rayfurthest from the optical axis, also called beam angle. Preferably, thisbeam angle is less than 5°, preferably 2.5°, more preferably 0.5°, oreven more preferably less than 0.1°.

In the presence of an eye, the incident laser beam is focused by thecrystalline lens of the eye towards the retina, which constitutes thefocal plane. The crystalline lens acts like a lens converging theincident rays towards the retina. This convergence is better as theincident rays are parallel to the optical axis. A low numerical apertureat the laser output 6 results in a laser beam at the entrance of theeye, and therefore incident to the crystalline lens, practicallycollimated, and therefore by a quasi-punctual focusing at the retina.Conversely, a high numerical aperture at the laser output 6 would resultin imperfect focusing by the crystalline lens, leading to an extendedspot.

In addition, inside the eye, the laser beam that crossed the crystallinelens can also be characterized by an ocular numerical aperture as itconverges towards the retina. This ocular numerical aperture depends inparticular on the diameter of the beam and on the reciprocal of thefocus of the eye. With a diameter of the laser beam at the pupil planeof the laser output 6 comprised between 4 mm and 8 mm, instead of aboutt mm for the systems of the state of the art, a large ocular numericalaperture is ensured. In addition, the fact that the laser beam incidenton the pupil is collimated ensures good focusing on the retina.

However, the lateral resolution (perpendicular to the direction ofpropagation of the beam, in x, y) of the laser impact defined by thefocal spot on the retina is proportional (for a perfect eye withcircular aperture) to the reciprocal of the ocular numerical aperture,while the depth extension is proportional to the square of thereciprocal of the ocular numerical aperture. As a result, the highocular numerical aperture allows limiting the dispersion of the laserbeam at the focal spot on the retina, and therefore containing the laserimpact. Furthermore, both lateral resolution and depth extension areproportional to the wavelength. The choice of a low wavelength (lessthan 690 nm, preferably less than 650 nm, and more preferably less than600 nm) allows further improving this containment.

However, this tow containment of the impact needs to overcome thedefects of the systems of the state of the art, which were hidden by thelarge size of their focal spot, and particularly the need to compensatethe ocular aberrations for the laser, improve the accuracy of thelocalization and allow a fine visualization of the location of the laserimpact both in lateral extension and in depth. To do so, the system hasan optical path 5 provided with various elements which will now bedescribed. Generally, the laser beam emitted by the photocoagulationlaser 1 propagates in an optical path 5 defined by the system andconnecting the upstream photocoagulation laser 1 to a downstream laseroutput 6, intended to be placed in front of the retina 4.

There are on this optical path 5:

-   -   an adaptive optics 9 positioned in the optical path 5 and        configured to modify the wavefront of the laser beam propagating        in the optical path 5 in order to compensate ocular aberrations        occurring up to the retina 4,    -   a position control loop 10 controlling a first actuator 14        positioned in the optical path downstream of the adaptive optics        in order to servo-control the position of the laser output 6        with respect to the retina 4 to be treated,    -   at least one imager 8 configured to acquire an image of the        retina 4 derived from the optical path 5.

With reference to FIG. 2 , an illustrative example is described below,whose different elements can intervene in various configurations asdesired, which are described from downstream of the optical path toupstream. It is of course understood that various modifications could bemade to the arrangement of the constituent elements of the system. Thesystem comprises a downstream laser output 6, intended to be placed infront of the retina 4. A first beam splitter S1 on the optical path 5allows deriving from the optical path an image of the retina 4. In thedescription, a beam splitter allows part of an incident light ray topass while reflecting another part in another direction, and can be ofany type, such as for example a half mirror.

There is, upstream of this first beam splitter S1, a shaping lens F1that allows in particular reducing the field angle, for example tovalues comprised between 15° and 25°, by choosing the appropriateequivalent focal length. A scanner SC1 constituting the first actuator14 allows controlling the displacement of the light beam in the opticalpath relative to the retina 4. The first actuator 14 allows scanning theentire central area of the retina, in particular because of the fieldangle which is still quite large.

Upstream of the first actuator 14 formed by the first scanner SC1, theoptical path 5 includes a beam splitter S2 which allows deriving animage of the retina 4 towards an optical coherence tomography OCTdevice, which is therefore connected to the optical path 5 by this beamsplitter S2. There is also on the optical path 5 a beam splitter S3 thatallows entering illumination fight in the optical path 5. Alter anothershaping len F2, there is a correcting element AO of the adaptive optics9.

The optical path 5 then includes upstream a shaping lens F3, then a beamsplitter S4 used to derive from the optical path 5 an image of theretina towards one or more imagers, upstream of the adaptive optics 9.Another beam splitter S5 on the optical path S5 allows deriving Sightfrom the optical path 5 to a wave front sensor ASO.

The optical path 5 then comprises a second actuator 7 placed upstream ofthe adaptive optics, configured to receive a laser displacement command,and to act on the laser beam to move the laser impact in three distinctdirections in space. More specifically, it is an actuator 7 forthree-dimensionally scanning the laser, making if possible to move thelaser impact at the retina 4 at least in the direction of propagation ofthe laser, noted z, but also in a plane perpendicular to the directionof propagation of the laser, defined by two distinct directions noted xand y.

To do so, the second actuator 7 comprises, as illustrated in FIG. 2 , ascanner SC2 positioned in the optical path and adapted to carry out adisplacement of the laser beam in a focal plane (x, y) perpendicular tothe propagation of the laser beam, and a connector L3 positioned in theoptical path 5 and movable in controlled translation in the direction ofthe laser beam. The connector L3 makes the junction between the opticaltransport fiber 3 and the optical path 5. The laser beam emitted by thephotocoagulation laser 1 passes from the optical transport fiber 3 tothe optical path 5 via this connector L3, which can therefore beconsidered as a source for the downstream elements. The scanner SC2 actsfor example by modifying the respective orientations of two mirrorsfacing each other, thus angularly shifting the laser beam that emergestherefrom.

The connector L3 is mounted on a motorized stage M2 receiving a laserdisplacement command and moving according to this command. Thedisplacement of the connector L3 along the optical path 5 causes amodification of its focus plane, which is reflected along the opticalpath 5 to result in moving the focal plane at the laser output 6, thatis to say the focus of the retina to be treated, in the direction ofpropagation of the laser beam. The user can thus finely modify the depthtreated by the laser impact, and for example modify this depth when itis observed that the laser impact would affect functional tissues. Thisfine depth adjustment is all the more useful, due to the strongcontainment of the laser impact obtained by the system, as the depth ofa laser impact no longer necessarily covers the entire area to betreated, and in that it can be necessary to carry out two laser impactsat different depths to treat the entire area to be treated.

In the case illustrated by FIG. 2 , the scanner SC2 is placed downstreamof the connector L3. However, it would be possible to invert theirorder. Furthermore, FIG. 2 shows the presence of a tens F4 which ispresent for the shaping of the laser beam at the outlet of the connectorL3. Its diameter and its focal length are chosen to allow the path oflaser beam satisfactorily between the constituent elements of thesystem. The disposition and the characteristics of this lens F4 aretherefore left to the appreciation of those skilled in the art, as arethose of the other tenses F1, F2 and F3.

The adaptive optics 9 is formed of several elements. As mentioned above,the adaptive optics 9 comprises a corrector element AO, used to correctthe wavefront of the laser beam. This correction to be made to thewavefront is determined by means of a sensor placed upstream of thecorrecting element AO, and configured to receive light from the opticalpath 5 whose nature depends on the chosen configuration.

In the example illustrated by FIG. 2 , the configuration of the adaptiveoptics 9 is called “Shack-hartmann”, and comprises a wavefront sensorASO, configured to receive tight from the optical path 5. In theillustrated case, the light is derived by means of the beam splitter S5placed on the optical path 5. The wavefront sensor ASO determines ameasurement representative of a shape of a wave surface of the Sighttraveling along the optical path 5. More specifically, the wavefrontsensor ASO decomposes a wavefront into elementary wave fronts and allowsdetermining for each elementary wave front its orientation. Themeasurement of these orientations allows, after integration, leadingback to the shape of the wave front.

Other configurations are possible. In particular, it is possible toprovide for an adaptive optics without a dedicated sensor, by exploitingfor example the imager 8 configured to acquire an image of the retina 4derived from the optical path 5. The analysis of the wave surface isthen made directly on the image obtained by this imager 8.

As explained above, the propagation of light in the eye reveals ocularaberrations which generate defects in this light, and particularly asregards its wave front. To better detect these alterations of thewavefront, a light source L1, associated with a lens, emits a lightwhose beam is similar to the one emitted by a point source, of smallextent. Preferably, this light has wavelengths comprised between 600 nmand 700 nm, in particular because this range highlights the mainaberrations, and is preferably still monochromatic. This light isreturned by a beam splitter S6 to the optical path 5, more specificallyto the beam splitter S3 already mentioned, and reaches the eye via thelaser output 6. An artificial point source is thus created on the retina4, which re-emits through scattering in all directions. Thebackscattered luminous flux propagates through the eye, then the opticalpath 5 up to the wavefront sensor ASO, which analyzes it in a pupilplane and determines the wavefront deformations, representative of theocular aberrations.

The measurements from the wavefront sensor ASO are received andprocessed by a processing module 20, preferably a real-time computer,configured to receive the measurement from the wavefront sensor, andcontrol the correcting element AO based on the measurement of thewavefront sensor, in order to compensate for the disturbances detected,in the typical case where the correcting element AO is a deformablemirror, the processing module 20 calculates the commands (for examplevoltages or intensities) to be sent to the deformable mirror, alsoplaced in a pupil plane. The surface of the deformable mirror thenchanges to compensate the measured wavefront deformations.

The action of the correcting element therefore allows compensating thedeformations for all the luminous fluxes arriving from the retina 4,such as for example those intended for imagers placed upstream of thecorrecting element, but also allows pre-modifying the luminous fluxesarriving from the upstream of the correcting element intended for theretina 4. These pre-mod if led luminous fluxes then have deformationsopposite to those they undergo during their path towards the retina 4 sothat, by reaching the retina 4, the ocular aberrations are compensated.Consequently, the light coming from the photocoagulation laser 1 has, atthe laser impact on the retina 4, a much improved quality and accuracy.Particularly, the containment of the laser impact is improved.

The correction provided by the adaptive optics also benefits the imagersdisposed upstream of the correcting element AO. Therefore, the systemalso comprises an imager 8 configured to acquire an image of the retina4 derived from the optical path 5, upstream of the adaptive optics 9.Preferably, the imager 8 is movable in controlled translation in thedirection of propagation of the light received by the imager 8. To doso, the imager 8 comprises an imaging camera IMA mounted on a motorizedstage M1 which allows, depending on received commands, moving thisimaging camera IMA. The beam splitter S4 allows deriving part of theluminous flux traveling through the optical path 5 towards the imagingcamera IMA, which allows obtaining a two-dimensional image of the areaof the retina facing the output 6 at a given focus depth. As mentionedabove, the imaging camera IMA can be used for the analysis of the wavesurface that allows controlling the correcting element AO of theadaptive optics 9 in a configuration without a dedicated wavefrontsensor.

By means of the motorized stage M1, the imaging camera IMA can be movedto modify the focus depth at the retina 4. It is then possible to imagethe retina at different depths, indeed, due to the significantcontainment of the laser obtained by means of the system, the depth of alaser impact no longer necessarily covers the entire area to be treated.The only information of the location of the laser impact is no longersufficient. It is then preferable to be able to make an imaging atdifferent depths so that the user can visualize the extent in depth ofthe area to be treated or of the area already treated.

The imager 8 can comprise, instead of the imaging camera IMA or inaddition, a camera STAB, called stabilization camera, for acquiring animage of the retina 4 derived from the optical path 5, upstream of theadaptive optics 9. This stabilization camera STAB is used to implement aregulation to servo-control the position of the laser output 6 withrespect to the retina 4 to be treated, in the example illustrated inFIG. 2 , the imager 8 comprises a stabilization camera STAB and animaging camera IMA. A beam splitter S7 receiving from the optical path 5part of the luminous flux circulating therein by means of the beamsplitter S4 allows supplying these two cameras with light. The imager 8is very upstream of the optical path, and requires great accuracy. Atthis stage, the field angle may have been reduced by the various crossedlenses F1, F2, F3 to a value less than 5°.

A light source L2 is provided for the illumination of the retina for theimager 8, that is to say here for the stabilization camera STAB and/orthe imaging camera IMA. This light source L2 preferably emits in awavelength comprised between 800 nm and 900 nm. The light emitted bythis light source passes through the beam splitter S6 it shares with thelight source L1 then meets the optical path 5 via the beam splitter S3.The emitted light meets the retina 4, and the light coming from theilluminated retina 4 goes up the optical path 5 via the adaptive optics9. The images acquired by the imager 8 thus benefit from the correctionprovided by the adaptive optics 9.

The imager 8 upstream of the adaptive optics 9 is therefore used toimplement a position regulation of the laser output 6 with respect tothe retina 4. More specifically, it is the output of the stabilizationcamera STAB that is used. The camera STAB transmits acquired images to aprocessing module 21, preferably a real-time computer which, from theseimages, determines a command of the first actuator 14 to move the lightbeam in the optical path relative to the retina 4 in order to stabilizethe position of the light beam, in particular by compensating theinvoluntary movements of the eye. The processing module 21 can forexample exploit an image of the photoreceptors and a correlation-typealgorithm to determine the movements of the retina 4, and determine acommand of the first actuator 14 so that the light beam follows thesemovements.

Thus, the first actuator 14 is controlled by the control loop 10 toservo-control the position of the laser output 6 with respect to theretina 4 to be treated. However, another measurement can be used tocontrol the first actuator 14. Preferably, the control loop 10determines a command of the first actuator 14 as a function, on the onehand, of a first measurement derived from the optical path 5 downstreamof the first actuator 14 and, on the other hand, of a second measurementderived from the optical path 5 upstream of the first actuator 14. Ifthis second measurement comes from the imager 8, the first measurementcomes from a large-field visualization subsystem, downstream of thefirst actuator 14.

The first beam splitter S1, placed downstream of the optical path 4,allows obtaining a wide visualization of the retina 4, exploited by alarge-field visualization subsystem 15. The field angle can thentypically be greater than 30°. The visualization subsystem 15 can beused both to acquire images Intended for the user, to servo-control theposition of the first actuator 14, and to project on the retina afixation target which can serve as a visual cue.

This visualization subsystem 15 comprises a light source and lens L4assembly emitting in a wide wavelength band, typically over a major partof the visible spectrum. The luminous flux thus emitted meets theoptical path 5 by the first beam splitter S1, via another beam splitterS8 and a shaping lens F5. This luminous flux then illuminates the retina4 in order to allow imaging the retina 4, in the other direction, theluminous flux coming from the retina 4 is derived from the optical path5 by the first beam splitter S1, passes into the shaping lens F5; and isdeviated by the beam splitter S8 in the direction of the visualizationimagers. Another beam splitter S9 allows directing the flux towards twovisualization imagers: a pupil imager PUP, which allows acquiring anddisplaying the image of the pupil, and a retina imager RET, which allowsacquiring and displaying the image of the retina. The retina imager RETallows implementing the position regulation of the scanner SC1 acting asa first actuator 14.

Thus, the control loop 10 comprises two control sub-loops. The firstcontrol sub-loop 11 comprises a first imager configured to receive animage of the retina derived from the optical path downstream of thefirst actuator 14 according to a first acquisition field. This firstimager is the retina imager RET, and the first acquisition field istherefore the angle field greater than 30°. The images acquired by theretina imager RET are transmitted to a processing module 22, typically areal-time computer, which determines a command for the first scannerSC1.

A second control sub-loop 12 comprises the imager 8 configured toreceive an image of the retina derived from the optical path upstream ofthe adaptive optics 9 according to a second acquisition field. It is thecontrol sub-loop comprising the stabilization camera STAB and theprocessing module 21. Preferably, the first field has a field angle atleast twice greater than a field angle of the second field, andpreferably greater than 10*. For example, if the first acquisition fieldis the angle field greater than 30°, the second acquisition field has anangle less than 15°, and preferably less than 10°. As the acquisitionfield is defined by the used imager, in the case presented above, thesecond acquisition field has an angle less than 5°, like that of theimager 8.

Preferably, also, the second control sub-loop 12 operates at a frequencygreater than the frequency of the first sub-loop 11. For example, thefirst sub-loop 11 can operate with a frequency less than 100 Hz, or evenless at 75 Hz, for example of 60 Hz, while the second sub-loop 12 canoperate with a frequency greater than 150 Hz, for example 200 Hz. Thefirst sub-loop 11 can thus be designated as a large-field orlow-frequency loop, and the second sub-loop 12 as a small-field orhigh-frequency loop.

It should be noted that the different control modules 20, 21, 22 can becombined together or in pairs, and it is possible for example that theirrespective functions are fulfilled by a single computer, preferablyreal-time computer. In addition, the system can comprise a visualizationscreen to which are sent the images acquired by the imagers such as theimager PUP, the camera IMA or the optical coherence tomography OCTdevice, so that the user of the system can visualize them.

Preferably, the optical coherence tomography OCT device is synchronizedwith other imagers in order to match the images acquired by the opticalcoherence tomography OCT device with the images acquired by anotherimager. This means that acquisition parameters of the optical coherencetomography OCT device are related to acquisition parameters of otherimagers. Preferably, the image acquired by the optical coherencetomography OCT device is synchronized on the image of the retinaobtained by the other imagers. This allows the user to be able tovisualize the same area on the OCT image and on the other images. Thisalso allows knowing the position of the laser shot on the OCT image.

Also preferably, the optical coherence tomography OCT device isconfigured to acquire an OCT image during the emission of the laser beamby the photocoagulation laser 1 so that the user can visualize in realtime the effect of the laser beam on the retina 4. The different imagersof course also allow visualization after and before the laser impact.

Thanks to the system described above, the size of the laser impactremains small, being more contained than the previous systems, and theuser can observe the laser impact in three dimensions and in real time.It is then possible to implement a displacement of the laser beamwithout interrupting it, in order to cover an area to be treated, unlikethe prior systems in which several distinct laser impacts were made, thepoint of laser impact being moved between two emissions of the laserbeam. It is also possible that the system is configured to emit thelaser beam in a discontinuous manner, but whose discontinuity is notrelated to the displacement of the beam. Preferably, the system isconfigured so that the second actuator 7 moves the path of the laserbeam while the photocoagulation laser 1 emits the laser beam, so thatthe laser impact on the retina moves between several locations in acontinuous manner. The displacement of the laser beam is preferablycarried out by the second actuator 7, but could alternatively or inaddition be carried out by the first actuator 14.

The invention is not limited to the embodiment described and representedin the appended figures. Modifications remain possible, in particularfrom the point of view of the constitution of the various elements or bysubstitution of technical equivalents, without thereby departing fromthe scope of protection of the invention.

The invention claimed is:
 1. A system for laser photocoagulation of aretina comprising: a photocoagulation laser comprising a laser sourceand an optical transport fiber, an optical path connecting aphotocoagulation laser to a laser output to be placed in front of theretina, so that a laser beam emitted by the photocoagulation laserpropagates along the optical path in a propagation direction, thepropagation direction being oriented from the photocoagulation laser tothe laser output, so that the photocoagulation laser is located upstreamthe laser output with respect to the propagation direction, said laserbeam having a wavefront, wherein the laser source is a laser sourceconfigured to emit in a wavelength comprised between 520 nm and 690 nmin the optical transport fiber, and the optical transport fiber has acore with adiameter less than 50 μm, the system for laserphotocoagulation comprising: an adaptive optics positioned in theoptical path and configured to modify the wavefront of the laser beampropagating along the optical path in order to compensate ocularaberrations occurring up to the retina, an imager configured to acquirean image of the retina based on light transported from the retina to theimager, the light following the optical path from the laser output to anextraction point, the light exiting the optical path at the extractionpoint, the extraction point being located in the optical path downstreamof the photocoagulation laser and upstream of the adaptive optics withrespect to the propagation direction, a first actuator being differentfrom the adaptive optics, the first actuator being positioned in theoptical path downstream of the adaptive optics with respect to thepropagation direction, said first actuator configured to control aposition of the laser output with respect to the retina, and a positioncontrol loop controlling the first actuator to servo-control theposition of the laser output with respect to the retina, on the basis ofan image of the retina derived from the optical path and acquired by theimager.
 2. The system according to claim 1, wherein a diameter of thelaser beam at a pupil plane of the laser output is comprised between 4mm and 8 mm.
 3. The system according to claim 1, wherein a beam angle atthe laser output is less than 5°.
 4. The system according to claim 1,wherein the position control loop comprises two position controlsub-loops: a first position control sub-loop comprising a first imagerconfigured to receive an image of the retina derived from the opticalpath downstream of the first actuator according to a first acquisitionfield, a second position control sub-loop comprising a second imagerconfigured to receive an image of the retina derived from the opticalpath upstream of the adaptive optics according to a second acquisitionfield.
 5. The system according to claim 4, wherein the first acquisitionfield has a field angle at least twice as great as a field angle of thesecond acquisition field, and/or the second control sub-loop operates ata frequency greater than a frequency of the first sub-loop.
 6. Thesystem according to claim 4, further comprising a visualizationsubsystem configured to receive an image of the retina derived from theoptical path downstream of the first actuator and acquire imagesaccording to the first acquisition field.
 7. The system according toclaim 1, further comprising a second actuator placed upstream of theadaptive optics, configured to receive a laser displacement command, andto act on a path of the laser beam in order to move a laser impact inthe retina in three distinct directions in space.
 8. The systemaccording to claim 7, wherein the system is configured so that thesecond actuator moves the path of the laser beam while thephotocoagulation laser emits the laser beam, such that the laser impactin the retina moves between several locations in a continuous manner. 9.The system according to claim 7, wherein the second actuator comprises:a scanner positioned in the optical path and adapted to perform adisplacement of the laser beam in a focal plane perpendicular to thepropagation of the laser beam, and a connector positioned in the opticalpath and movable in controlled translation in the propagation directionof the laser beam.
 10. The system according to claim 1, wherein theimager comprises a camera configured to acquire an image of the retinaderived from the optical path upstream of the adaptive optics, thecamera being movable in controlled translation in the direction ofpropagation of the light received by said camera.
 11. The systemaccording to claim 1, wherein the adaptive optics comprises: acorrecting element, disposed in the optical path, and a processingmodule configured to control the correcting element based on an analysisof a wave surface of a light traveling along the optical path.
 12. Thesystem according to claim 11, wherein the analysis of the wave surfaceis provided from a wavefront sensor configured to receive light from theoptical path upstream of the correcting element and to determine ameasurement representative of a shape of a wave surface of the lighttraveling along the optical path, or the imager comprises a cameraconfigured to acquire an image of the retina derived from the opticalpath upstream of the adaptive optics, or the analysis of the wavesurface is provided from an image acquired by the imager comprising acamera configured to acquire an image of the retina derived from theoptical path upstream of the adaptive optics.
 13. The system accordingto claim 1, comprising an optical coherence tomography device connectedto the optical path downstream of the adaptive optics.
 14. The systemaccording to claim 1, wherein the laser source is a single-mode fiberlaser source.